Heretofore, the nonvolatility of recorded information, i.e., the state in which recorded information is retained even if the power supply is turned off, has been achieved by using a magnetic tape drive, a hard disc drive, an optical disc drive, a magneto-optical disc drive, or the like. On the contrary, in recent years, experiments have been actively conducted in which nonvolatility is achieved by using a semiconductor-based solid-state component. Among these, for example, flash memory, FeRAM, and the like have already been put to practical use. These memory devices do not contain any movable machine parts, such as those needed in a magnetic tape drive and the like, and are smaller in size and consume lower amounts of electric power, making them excellent candidates as practical memory device.
A flash memory is summarized in the first document, Hitoshi KUME, “Flash memory technology,” Applied physics, Vol. 65, No. 11, (1996): pp. 1114 to 1124. An FeRAM is summarized in the second document, Elliott M. Philofsky, “FeRAM The Ultimate Memory,” The Journal of The Institute of Electronics, Information and Communication Engineers, Vol. 80, No. 2, (1997), pp. 169 to 175.
However, flash memory devices and FeRAMs have the following problems.
The currently used flash memory devices need a high operating voltage, and, according to the first document, the maximum internal voltage thereof is 12 V. This is extremely high compared to the operating voltage of general DRAMs and LSIs, which can be-operated by a voltage of 3 to 4 V. In addition to this problem, the currently used flash memory devices take 1 millisecond to 1 second to rewrite data. This is very troublesome for users, when it is necessary to frequently rewrite data.
According to the second document, FeRAMs have an internal voltage that is not more than 5 V, which is lower than that of a flash memory. The access time thereof is as short as 250 nanoseconds. However, FeRAMs have a drawback in that the switching properties of the ferroelectric capacitor are susceptible to temperature changes. Furthermore, although high-temperature annealing is necessary to fabricate an FeRAM, the ferroelectric layer thereof contains Pb, Bi, and like metals with low melting points as constituent components, and these elements are therefore diffused into the substrate.
Because of these problems, in recent years, research and development relating to nonvolatile recording devices that are collectively called “MFS transistors” and that differ from flash memory and FeRAM devices has been actively conducted. MFS transistor technology is summarized, for example, in the third document, Yasuo TARUI, “Trend of development and future of ferroelectric memories,” The Journal of The Institute of Electronics, Information and Communication Engineers, Vol. 77, No. 9, pp. 976 to 979. In this device, a ferroelectric is arranged in the gate electrode of an ordinary MOS transistor, wherein a nonvolatile memory is obtained by varying the channel conductance in the transistor by changing the polarization direction of the ferroelectric.
Such an MFS transistor employs the following structure. Ordinarily, it is difficult to dispose a ferroelectric directly onto an Si substrate because problems arise such as the diffusion of elements, etc. Therefore, in most cases, an MFIS structure, in which an insulator film (insulator) that also serves as a diffusion prevention layer is inserted between the Si substrate and the ferroelectric film, or an MFMIS structure, in which a floating gate electrode is further incorporated therein, is employed.
However, transistors having the MFMIS structure, in fact, also pose several problems. For example, when the ferroelectric film is switched by applying voltage to the gate, even after the voltage is removed, a depolarization field that is generated by polarization still exists in the ferroelectric film. Therefore, the ferroelectric film is constantly subjected to a force in the direction in which the stored polarization is cancelled, and this makes it difficult to stably maintain polarization. Furthermore, due to this depolarization field, current gradually flows from the semiconductor substrate or a controlling gate electrode disposed on the semiconductor substrate to the floating gate electrode and ferroelectric film. The current that flows thereto will gradually compensate the charge of the floating gate electrode that is generated by the polarization of the ferroelectric film, finally resulting in the loss of stored information. In other words, information will become undesirably volatile and this prevents the transistor from serving as a nonvolatile memory device.
The duration of time that a nonvolatile memory device can retain information is referred to as retention time. If information volatilizes as described above, it becomes impossible to achieve a satisfactory retention time. Note that the average retention time guaranteed in flash memory devices is currently 3×108 seconds, which corresponds to 10 years.
With this drawback in view, several experiments relating to MFMIS transistors have been conducted in order to prolong the retention time thereof by controlling the leakage current. For example, the fourth document, “M. Takahashi et al., Jpn., J. Appl. Phys., Vol. 10 (2001), pp. 2923 to 2927” reports a calculation that the retention time can be prolonged to 1×1012 seconds by structuring an MFMIS in such a manner that the insulator film is inserted between the control gate electrode and the ferroelectric film of the MFMIS transistor. Alternatively, an MIFIMIS structure is proposed in which an insulator film is also inserted between the floating gate electrode and the ferroelectric film to reduce the leakage current.
In MOS transistors, in order to prevent leakage current, instead of the insulator film made of silicon dioxide, tetranitrogen trioxide, silicon nitride/oxide, or the like that has heretofore been arranged between the gate electrode and the semiconductor substrate, the use of an insulator film made of a material that has a higher dielectric constant is being considered. Such an insulator film having a high dielectric constant is generally called a “high-dielectric-constant film” or a “high-k film.” The use of such a film increases the physical thickness of the insulator film, preventing the leakage of current. Examples of excellent candidates for materials for the insulator film include ZrO2, Al2O3, La2O3, PrO3, Gd2O3, Y2O3, etc.
Similarly, hafnium dioxide (HfO2) and like hafnium oxides, hafnium silicon oxides (HfSiOx), hafnium silicate aluminates (HfSiAlOx), hafnium nitride oxide (HfON), and the like are also excellent candidates.
Hence, introducing the high-k films used in MOS transistors also into MIFIMIS transistors in order to control leakage current is being considered. Hereunder, the properties of the insulator film required in MOS transistors and MIFIMIS transistors will be explained.
FIG. 11 shows the figure of merit required in MOS transistors and MFMIS transistors. In other words, it shows the relationship between SiO2 equivalent film thickness (EOT: equivalent oxide thickness) and leakage current density (J). According to this figure, in MOS transistors, the EOT in the year 2001 was 1.6 nm and it is expected to decrease to 0.8 nm by the year 2005. This is because, in MOS transistors, a higher capacitance will be needed since the area of the gate will decrease as devices are further miniaturized, and therefore the insulator film should be made extremely thin. However, this increases leakage current density, i.e., from 0.1 A/cm2 to 1×103 A/cm2.
In the MFMIS transistor, however, its performance as a nonvolatile device is the most important. The leakage current density thereof was 1×10−8 A/cm2 in the year 2001 and it should be 1×10−13 A/cm2 by the year 2005 in order to retain stored information. On the other hand, although the thinner EOT is better, from the viewpoint of a nonvolatile device, 5 nm will be satisfactory in the year 2005.
As described above, heretofore used MOS transistors and MFMIS transistors have quite different requirements for the insulator film performance. Therefore, it is insufficient to apply high-k films, whose introduction is being considered in MOS transistors, to MFMIS transistors to obtain an MIFIMIS structure.
The present invention aims to solve the above problems and to provide a semiconductor device in which the amount of leakage current can be decreased in an MIFIMIS and like structures wherein an MFMIS structure is provided with an insulator film, and a fabrication method thereof.